A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ            NA                                              (        1        )            where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.
EUV radiation may be produced using a plasma. A radiation system for producing EUV radiation may include a laser for exciting a fuel to provide the plasma, and a source collector apparatus for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as particles of a suitable material (e.g., tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. The source collector apparatus may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source.
If contamination were to be present within an EUV lithographic apparatus then that contamination could introduce errors into patterns projected by the EUV lithographic apparatus onto substrates. For example, a contamination particle may become attached to a patterned area of a patterning device in the EUV lithographic apparatus, thereby obscuring part of the patterned area and introducing an error into the projected pattern. It is therefore desirable to minimise contamination, particularly particulate contamination, in the EUV lithographic apparatus. One way in which this may be done is to measure particulate contamination of surfaces of components that are to be used to assemble an EUV lithographic apparatus. A component may for example be rejected and/or cleaned if the measured particulate contamination of its surfaces is too high.
A known principle of contact cleaning surfaces by removing particles from a given surface makes use of tacky materials assembled in devices such as floor mats, contact rollers or manual rollers. Various tacky materials used in such particle removing devices include polyurethanes, latex, or soft rubbers.
The use of such tacky materials is also known in particle detection methods being used for transferring the particles onto the surface of the tacky materials and analyzing the surface with microscopy. The tape lift method is for example an analytical method that use tacky materials like latex tapes.
However, such tacky materials standardly contain adhesives, chemicals, release agents or polymer components that can be released. Such materials may leave minimal residues at contact, however for EUV applications even such low residue level may be unacceptable. US 06449035 B1 describes for example materials which are said to leave no residues. However the polymer materials disclosed therein are materials which still may leave to some extent (mono)layers of silicones on the contacted surface. Although only detectable with advanced surface analyses like XPS, even such minor amount of residues can have detrimental effects when they are present in a EUV lithography scanner, an EUV radiation source or other apparatus. This is because even minor amounts of residue may influence the contamination buildup on optics, thereby changing their output, or have influence on the wetting characteristic in coating processes.
Most of the tacky materials typically used in contact cleaning or in the particle detection methods leave residues such as (mono)layers of organic materials, silicone or refractory components, which are not acceptable for the high cleanliness requirements in EUV lithography.